macsec: fix UAF bug for real_dev

In the Linux kernel, the following vulnerability has been resolved:

macsec: fix UAF bug for real_dev

Create a new macsec device but not get reference to real_dev. That can

not ensure that real_dev is freed after macsec. That will trigger the

UAF bug for real_dev as following:

==================================================================

BUG: KASAN: use-after-free in macsec_get_iflink+0x5f/0x70 drivers/net/macsec.c:3662

Call Trace:

...

macsec_get_iflink+0x5f/0x70 drivers/net/macsec.c:3662

dev_get_iflink+0x73/0xe0 net/core/dev.c:637

default_operstate net/core/link_watch.c:42 [inline]

rfc2863_policy+0x233/0x2d0 net/core/link_watch.c:54

linkwatch_do_dev+0x2a/0x150 net/core/link_watch.c:161

Allocated by task 22209:

...

alloc_netdev_mqs+0x98/0x1100 net/core/dev.c:10549

rtnl_create_link+0x9d7/0xc00 net/core/rtnetlink.c:3235

veth_newlink+0x20e/0xa90 drivers/net/veth.c:1748

Freed by task 8:

...

kfree+0xd6/0x4d0 mm/slub.c:4552

kvfree+0x42/0x50 mm/util.c:615

device_release+0x9f/0x240 drivers/base/core.c:2229

kobject_cleanup lib/kobject.c:673 [inline]

kobject_release lib/kobject.c:704 [inline]

kref_put include/linux/kref.h:65 [inline]

kobject_put+0x1c8/0x540 lib/kobject.c:721

netdev_run_todo+0x72e/0x10b0 net/core/dev.c:10327

After commit faab39f63c1f ("net: allow out-of-order netdev unregistration")

and commit e5f80fcf869a ("ipv6: give an IPv6 dev to blackhole_netdev"), we

can add dev_hold_track() in macsec_dev_init() and dev_put_track() in

macsec_free_netdev() to fix the problem.